This invention relates to the protection of surfaces from excessive oxidation using a aluminum-containing protective coating and, more particularly, to the prevention of excessive oxidation of the protective coating.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor and fan. In a more complex version of the gas turbine engine, the compressor and a high pressure turbine are mounted on one shaft, and the fan and low pressure turbine are mounted on a separate shaft. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion and exhaust-gas temperatures. The maximum temperature of the combustion gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine, upon which the hot combustion gases impinge. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to about 1900-2150xc2x0 F.
Many approaches have been used to increase the operating temperature limits of turbine blades, turbine vanes, and other hot-section components to their current levels. For example, the composition and processing of the base materials themselves have been improved, and a variety of solidification techniques have been developed to take advantage of oriented grain structures and single-crystal structures. Physical cooling techniques may also be used.
The surfaces of the articles may be protected with an aluminum-containing protective coating, whose surface oxidizes to an aluminum oxide scale that inhibits further oxidation of the surfaces. However, the aluminum oxide scale is relatively permeable to oxygen. During service, oxygen diffuses from the environment and through the aluminum oxide scale to the underlying aluminum-containing protective coating, whereupon more aluminum oxide is formed. This formation of aluminum oxide is good to a point, but the formation of too thick an aluminum oxide scale may lead to spallation of the aluminum oxide scale, consumption of the aluminum in the aluminum-containing protective coating, and the loss of protection of the underlying substrate. Excessive diffusion of oxygen may also lead to excessive oxidation of the underlying substrate.
There is therefore a need for an improved approach to the aluminum-containing protective coatings on surfaces of materials used at high temperatures, such as nickel-base superalloys. The present invention fulfills this need, and further provides related advantages.
The present invention provides a protected article that is protected both by an aluminum-containing protective coating and a layer that is highly impervious to oxygen. The aluminum oxide scale forms on the aluminum-containing protective coating to protect the underlying substrate. The oxygen barrier layer inhibits further diffusion of oxygen to the aluminum-containing protective layer, so that it does not form too thick an aluminum oxide scale, which is prone to failure by spallation, and is not consumed too rapidly. The result is a longer-lived protection of the underlying article.
A protected article includes a substrate and a protective structure overlying a surface of the substrate. The protective structure comprises a protective coating comprising aluminum and overlying the surface of the substrate, and an oxygen barrier layer comprising an iridium alloy having at least about 70 percent by weight iridium and overlying the protective coating. The iridium alloy preferably has no more than about 90 percent by weight iridium. The oxygen barrier layer preferably has a thickness of from about 5 micrometers to about 50 micrometers.
The substrate is preferably a nickel-base alloy such as a nickel-base superalloy. The protective coating may be a diffusion aluminide such as a simple diffusion aluminide, an example being a nickel aluminide, or a complex diffusion aluminide such as a platinum aluminide. The protective coating may instead be an overlay coating such as an MCrAlX overlay coating. A ceramic thermal barrier coating may overlie the protective coating and the oxygen barrier layer.
In this layered system, the aluminum-containing protective coating oxidizes to form an aluminum oxide scale that protects the substrate article from excessively rapid oxidation. The iridium-containing oxygen barrier layer, which may be quite thin because of the low permeability of oxygen in high-iridium alloys, inhibits the diffusion of oxygen to and through the aluminum oxide scale to the underlying protective coating. The result is that the aluminum oxide scale does not grow too thick or too rapidly, so that it may continue to protect the surface for extended periods of time.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.